For most applications of ceramics, it is desirable to increase the strength of a ceramic body while decreasing its weight or density. One method to decrease the weight of a ceramic body is to introduce porosity into the body. The introduction of pores into a ceramic matrix, however, generally causes a decrease in strength of the resulting ceramic body. This effect is thought to be due to the creation of micromechanical stress concentrations in the matrix created by the presence of the pores, which results in a decrease in the overall strength of the ceramic. Applications of porous ceramics include use in medicine as artificial body joints, where weight and biocompatibility is a critical factor. In another medical application, porous ceramics may be used as bone substitutes where the porosity of the material facilitates tissue infiltration. Porous ceramics may also be used in a variety of other applications, for example as microfilters, or as electronic elements. Porosity alters the dielectric properties of ceramics, so that a porous ceramic can function as an insulator and can be used, for example, as a substrate for computer hardware such as microchips. Also, porous ceramics may be infiltrated with another material such as a metal, to form composites having preselected thermal and mechanical properties, for example, rapid heat dissipation and higher fracture toughness or, can be infiltrated by polymers to form piezoelectric ceramics which can function as transducers for use in sonar equipment. It is necessary for all these applications that the porous ceramic body retain sufficient strength to withstand the stresses of the particular application.
It has been observed that the decrease in strength of porous ceramic materials is exponential with increasing porosity content. Theoretical studies claim that strength will not show an exponential decay if the pores are spherical in shape and smaller in size. Evans, et al., J. Amer. Ceram. Soc. 62(1-2):101-106 (1979). Bertoletti (Bertoletti et al,, J. Amer. Ceram. Soc. 50:558 (1967)) has investigated the effect of spherical pore size and volume fraction porosity on the strength of sodium borosilicate glass matrices incorporating nickel spheres to form the pores. Since the coefficient of thermal expansion of the nickel is larger than that of the glass, and because of a lack of bonding between the nickel spheres and the glass, the nickel spheres were expected to develop pores upon cooling. Bertoletti observed that strength increased with an increasing volume of porosity when the size of nickel spheres were less than 10 .mu.m. The use of nickel spheres to form pores, however, does not provide for widespread applicability for the following reasons: (1) Nickel cannot be used to produce ceramic materials that require high sintering temperatures. Different ceramic materials may require different sintering procedures. Borosilicate glass, for example, may be liquid phase sintered at a relatively low temperature. Liquid phase sintering is a densification process which involves the formation of a viscous liquid phase at the firing temperature and elimination of pores by the flow of this liquid. Other ceramic materials, such as aluminum oxides, on the other hand, require higher temperature solid phase sintering in which the elimination of pores is achieved during densification by the transfer of matter from the particles or grain boundaries through the diffusion of ions. (2) When smaller pore size is desired, and smaller nickel spheres are used (approximately 5 .mu.m), as Bertoletti suggests, the nickel spheres may bond to the glass and not separate upon cooling due to relatively small thermal contraction, so that no pores are formed (this phenomenon may account for the increased strength of the "porous" samples observed by Bertoletti using 5 to 10 .mu.m spheres). (3) The nickel may oxidize and subsequently bond with the matrix phase, also preventing pore formation. (4) Use of nickel increases the density of material.
Biswas (Biswas et al., Trans. J. British Ceram. Soc., 79, pp. 1-5 (1980)), using a glass matrix, niobium-doped lead zirconate titanate (PNZT) obtained relatively small sized pores (2 to 3 .mu.m) by changing the sintering conditions; i.e., by lowering the sintering temperature and altering the sintering time. Biswas also used an excess of lead oxide (PbO) during liquid phase sintering. Although it might be possible to obtain small spherical pores under certain conditions simply by changing the sintering conditions, as did Biswas, it is impossible using such techniques to reach a high-volume fraction of porosity while maintaining a spherical pore shape. Volume percent porosity is the ratio of the volume of pores (voids) to the total bulk volume of the porous sample. In addition, in applications where larger pores are desired, this processing method would not be effective to tailor pore size and shape. Thus, it is difficult to adequately control porosity by merely changing the sintering parameters. In addition, a comparison of strengths of a porous sample with its dense form might be hindered using Biswas' methods because the composition and distribution of the lead oxide in the porous and dense samples might not be the same. The oxide-rich liquid phase formation has a low melting temperature (890.degree. C.). Since sintering of the dense samples occurs at higher temperatures (1200.degree. C.) where the lead oxide tends to melt and evaporate, the composition of the dense sample may differ from that of the porous sample. It might also be expected that some of the lead oxide liquid would crystalize during cooling from the temperature of 1100.degree. C. used by Biswas in the fabrication of their porous samples, such that the distribution of the lead oxide within the matrix of the samples may also differ. Therefore, one would expect the measurements of the mechanical strength of the dense and porous samples in Biswas to differ not only because of the presence of pores, but also as a result of differences in phase distribution and composition between the dense and porous samples.
It has long been known that the fracture strength of ceramics is decreased by the severity of defects or "flaws," such as pores that exist throughout the ceramic material. For the most part, fractures in ceramics originate from surface flaws. Pores near the surface of the body can combine with these surface flaws and contribute to the decrease in strength. In addition, internal pores that are too close together, for example a cluster of pores, may function as a large flaw, creating a combined decrease in strength similar to that of a surface flaw. In general, the larger the pore size and the more irregular the shape (e.g., deviation from spherical), the larger the effective surface flaw size. The effect of surface flaws must be taken into account to decrease the statistical error in strength measurements of a ceramic sample.
The effects of the presence of pores on strength may be minimized by separating the pores by an optimum distance from one another. However, it has not been previously possible to closely control pore spacing and distribution to provide a porous ceramic that both minimizes the deterimental effect of surface flaws on strength in combination with pores near the surface, and maximizes the strength of the porous ceramic. Thus, until the present invention, a process for producing porous ceramics that provides the ability to control pore size, shape and spatial distribution of pores, while maintaining strength, has not been achieved.